FIG. 22 is a circuit diagram illustrating the structure of a conventional drive circuit 130. In FIG. 22, drive circuit 130 includes a differential amplifier 131, a P-type field-effect transistor (hereinafter referred to as a P-type transistor) 132 and a constant-current circuit 133. P-type transistor 132 is connected between a power supply potential VDD line and an output node N132. Constant current circuit 133 allows a constant current to flow from output node N132 to a ground potential GND line. Differential amplifier 131 has a non-inverting input terminal receiving the potential VI at an input node N131, an inverting input terminal receiving the potential VO at output node N132, and an output terminal connected to the gate of P-type transistor 132.
When the output potential VO is higher than the input potential VI, the output potential of differential amplifier 131 is increased, the electric current flowing into P-type transistor 132 is decreased, and the output potential VO is decreased. When the output potential VO is lower than the input potential VI, the output potential of differential amplifier 131 is reduced, the current flowing into P-type transistor 132 is increased, and the output potential VO is increased. Thus, VO=VI.
Unfortunately, since conventional drive circuit 130 has a direct feedback from the output potential VO to differential amplifier 131, the load capacitance may vary when it is large, or oscillation may occur when the input potential VI varies.